The present invention is directed to electrical test probe tips and, more particularly to resistive test probe tips.
Electrical test probes are used to provide an electrical connection between electrical components and testing instruments such as oscilloscopes and other measuring, monitoring, diagnostic, and signal processing instruments. An electrical test probe generally consists of a probing head, a cable, and a testing instrument connector. The probing head may have an integral or replaceable electrical test probe tip that is suitable for making an electrical contact with electrical components. The testing instrument connector is suitable for connecting to a testing instrument. The probing head is attached to a first end of the cable and the testing instrument connector is attached to the opposite end of the cable. The probe head circuitry represented in FIGS. 2, 3, and 6 are meant to be broad generalizations of probe heads. It is recognized that probe heads in general may have some parasitic capacitance and inductance.
A perfect test probe tip would have a frequency response in which the voltage in was equal to the voltage out. As shown in FIG. 1, this frequency response would be perfectly flat, like trace A.
Traditional test probe tips 30 are generally a single piece of metal that may or may not have a shaped probing end or point of contact 32. The form (e.g. the pointed tip) and strength of these traditional test probe tips 30 make them extremely useful for probing electrical components. As shown in FIG. 2, these traditional test probe tips 30 have some inherent inductance 34 and at least some stray capacitance 36 between the point of contact 32 and ground 38. A traditional metal test probe tip has a frequency response such as trace B (FIG. 1). This frequency response tends to have a relatively high peak, but then falls off sharply.
The evolution of electronic circuitry and higher bandwidth signals necessitated new probe tips with frequency responses closer to the perfect frequency response. As shown in FIG. 3, to counteract the resonance between the inductance 34 of the tip 30 and the stray capacitance 36, it has been known to add a resistor 40 (which may have leads) just before the point of contact 32. A test probe tip with an added resistor has a frequency response such as trace C (FIG. 1). This frequency response does not have the relatively high initial peak associated with traditional metal probe tips. Adding the resistor 40 and leads (leaded resistor 40) is problematic because it adds unwanted length to the tip 30. In use, this type of probe tip usually must be soldered into place, as its leads are too soft for a browsing (quick touch) type of probing of electrical components. To use this type of probe tip for quick touch browsing the probe tip would have to have reinforcement that would add parasitic capacitance.
When a capacitor (or low pass filter) is added between differential probe tips or a capacitor is added between the single ended probe tip and ground, the effect is to “roll off” the frequency response. As shown in trace D of FIG. 1, although this would bring down the peak of the frequency response, it would also reduce the bandwidth performance of the tip.
Extrusion and pultrusion are manufacturing methods. Using extrusion, material (e.g. plastic, composites, resins, or metals) is pressurized and forced (pushed) through an opening of a particular shape. Using extrusion, the finished product will be larger than the die opening due to the pressure flowing through the die. Using pultrusion, material is “pulled” or drawn through an opening of a particular shape. Using pultrusion, the finished product will be smaller than the die opening due to the pulling (stretching) of the material. Pultruded products tend to be stronger than extruded products. As an example of pultrusion, fibers (e.g. fiberglass) may be impregnated with liquid resin, carefully formed, and pulled through a heated die by powerful equipment. A fully cured and solid composite profile exits the die. The resulting product may be cut, shaped, and/or machined.
As a manufacturing process, pultrusion has many advantages including ease of automation and cost-effectiveness. Pultruded products also have many advantages. For example, all pultruded profiles have continuous cross section, but they can have a variety of shapes, sizes, colors, fabrication options, and protective finishes. Pultruded products can be tailored to provide high performance and cost advantages over materials such as metals, wood, and extruded thermoplastics. As compared to metals, pultruded composites offer weight reduction, thermal insulation, superior corrosion and chemical resistance, greater strength, and reduced expansion and contraction with temperature (CTE).
U.S. Pat. No. 5,139,862 to Swift et al. (the “Swift reference”), the disclosure of which is incorporated herein by reference, is directed to an electronic device for conducting electric current that has two contacting components at least one of which is a nonmetallic electronic contact in the form of a pultruded composite member made of a plurality of small generally circular cross section conductive fibers in a polymer matrix. The fibers are oriented in the matrix in a direction substantially parallel to the axial direction of the pultruded composite member and are continuous from one end of the member to the other to provide a plurality of electrical contact points at each end of the member.